1. Field of the Invention
The present invention relates to a coherent demodulating device and more especially means for recovering a carrier wave in the modulated signal that the demodulating device receives.
The demodulating device is included in a receiver of a digital transmission system, e.g. for digital communications between microwave relay stations and/or by satellite.
2. Description of the Prior Art
In a transmitter of a transmission system, a data digital signal modulates a radio carrier. The modulation employed can be a multi-phase modulation or a multi-amplitude modulation. Multi-phase modulation is a phase modulation with N states or symbols, where N is an integer equal to a power of 2; e.g. when N is equal to 4, 8 or 16, the phase modulation is designated by the abbreviation 4-PM, 8-PM, or 16-PM. The multi-amplitude modulation is known under the initials QAM, meaning "quadrature amplitude modulation" and combines two quadrature carriers modulated respectively, e.g. to 2, 4 or 8 amplitude states or levels; thus a multi-amplitude modulation offers, e.g. 4, 16 or 64 states or symbols corresponding to the initials 4-QAM, 16-QAM, 64-QAM.
Thus whatever the modulation employed, this makes use of a modulation by two component signals, derived from the digital signal to be modulated, modulating two phase quadrature waves derived from the same carrier wave. The carrier frequency is greater than the transmission frequency of the symbols, also referred to as clock frequency 1/T, T being the clock period. The modulated digital signal is generally transposed to the transmission frequency in the transmitter of the transmission system. In the receiver, frequency-transposing and preamplifying means receive the transmitted signal and apply the intermediate-frequency multi-symbol modulated signal to the demodulating device.
The two parameters necessary to restore the digital signal transmitted by a transmitter are the phase and frequency of the carrier wave.
The known carrier recovering circuits use phase servoing techniques. These circuits are obtained in the shape of an analog phase locked loop (PLL) including a phase comparator, a filter and a voltage-controlled oscillator. The drawbacks involved in these embodiments are numerous. From the point of view of performances, the compensation for a frequency difference between the demodulation carrier wave frequency supplied by the voltage-controlled oscillator and the frequency of the modulation carrier wave in the transmitter is only accomplished as counterpart to a phase error proportional to the frequency difference between the carrier waves and inversely proportional to the loop gain. As the phase error has quite considerable consequences on the performances of the transmission system, it should be limited to low values, typically a few tenths of a degree, which requires increasing the loop gain. It is however impossible to increase this gain arbitrarily, on grounds of loop stability.
Another consequence induced by this phase error is the limitation of the acquisition frequential range to frequency values compatible with an acceptable static phase error.
From the point of view of the use of such a phase loop, exacting adjustements are necessary in order to obtain the required performances. Moreover a modification to one of the loop parameters or transmission system parameters, such as modulation speed, requires a complete redefinition of the whole loop.